When a looped path is created in a network such as Ethernet (registered trademark), drawbacks such as a broadcast storm may occur.
To deal with a network loop, there is a technology known as Spanning Tree Protocol. Spanning Tree Protocol is a technology that logically cuts a loop by disabling some of links in a network. However, Spanning Tree Protocol has its own drawback such that a detection error may occur when a control frame for detecting a loop is lost. Furthermore, since Spanning Tree Protocol disables some of links in the network, there is a drawback such that the band of the disabled link becomes unavailable.
There is another technology known as a link aggregation. In the link aggregation, a plurality of links are collectively treated as a single logical link. For example, even when a physical loop is created by connecting two switches with a plurality of links, the plurality of links are treated as a single link. Thus, logically, no loop exists, and frames would not travel back and forth between the switches. However, the link aggregation has a drawback such that appropriate use of the plurality of links may become a difficult task, and sometime load may concentrate in a certain link. Furthermore, when a new switch is added to a network and a logical link is set up between the new switch and other existing switch by the link aggregation, there may be a case where new setting is performed on the existing switch. In such a case, unless the existing switch is appropriately set up for the link aggregation, unfavorable side effects may occur in other parts of the network.
Furthermore, there is still another technology known as End Host Mode. In End Host Mode, for example, when a switch is installed between an external network and a plurality of servers, network side ports are set not to forward frames among the network side ports, thereby avoiding a creation of a loop. Furthermore, each of server side ports is associated with one of the network side ports in advance. When a frame is received at one of the server side ports and a transmission port corresponding to a destination is unknown, the frame is output to one of the network side ports, which is associated with the port that received the frame. This enables to deal with the foregoing drawbacks of Spanning Tree Protocol and the link aggregation. However, in End Host Mode, there may be a drawback such that communication paths are mostly fixed since each of the server side ports is associated with one of the network side ports.
Furthermore, with regard to a network switch, there is the following conventional technology. More specifically, a communication system includes a first group of switching units and a second group of switching units, and a network is formed by connecting those switching units. The network is divided into at least two virtual sub-networks, and each of the sub-networks forms a spanning tree. Each of the switching units allocates packets arrived at a certain port to a predetermined virtual sub-network. However, this technology assumes a special system that uses a plurality of switching units, and may not be used to deal with the foregoing drawbacks of End Host Mode.
Japanese National Publication of International Patent Application No. 2008-546332 discloses a related technology.